There is a need for simple diagnostic tests for common diseases which can be carried out by untrained personnel. Such tests facilitate home or doctor's office testing as opposed to more complicated procedures which require that the analysis be carried out in an outside reference laboratory. A common format for these tests is the immunostrip format. Typically, this format involves a matrix of a material through which a fluid test sample can flow by capillarity. The matrix, typically in the form of a strip, contains an analyte specific antibody which bears a detectable label so that the presence and/or concentration of the analyte in the test fluid can be determined by detection of the signal emitted from the detectable label. A classical format for such a device, sometimes referred to as an immunochromatographic strip, is illustrated by FIG. 1. Referring to FIG. 1, strip 10, bears a labeled antibody specific for the analyte under investigation in zone 13 which binds with the analyte in the fluid test sample applied to the wicking zone 12 of the strip 10 and flows along the strip to form an immunocomplex which further migrates due to capillary action through the capture zone of the strip 14 and the optional detection zone 16. In the capture zone 14 there is immobilized the analyte or a derivative thereof which is immunoreactive with the labeled antibody and is able to capture labeled antibody which has not reacted with analyte in the fluid test sample. The signal from the labeled antibody captured in the capture zone is measured and related to the concentration of analyte in the test fluid in an inverse relationship since the greater the concentration of analyte in the test sample, the amount labeled antibody which is unbound and thereby free to specifically bind with analyte immobilized in the detection zone is diminished. Detection zone 16 is optional but can contain immobilized anti-mouse IgG to bind the analyte/labeled binding partner complex and thereby serve as a means for verifying that the test has been carried out correctly.
A problem with this sort of test device involves the tendency of labeled antibody and its conjugate to engage in non-specific binding (NSB) with the matrix material forming the strip. When such non-specific binding takes place, the labeled antibody binds to the matrix material before it reaches the capture zone and the assay fails because the movement of labeled antibody is either completely stopped or diminished such that the signals in the capture zone and detection zone are greatly reduced.
In order to correct this bias, the strip can be treated with a blocking solution such as 1% casein in phosphate buffered saline (PBS), washed with water and dried after deposition of the reagents onto the capture and collection zones. This blocking step, however, is problematic since there is required extensive development effort to optimize the blocked system.
In U.S. Pat. No. 5,451,507 there is described the preparation of a blocked nitrocellulose membrane for use as an immunochromatographic strip in which the nitrocellulose membrane is incubated in a solution of 1 mg/mL bovine IgG in sodium sulfate buffer for 30 minutes before being incubated with glutaraldehyde and bovine IgG. This reference also mentions the desirability in some instances of including from about 0.05 to 0.5 weight percent of a non-ionic detergent with the fluid test sample.
It would be desirable, and it is an object of the present invention to provide a means for reducing or eliminating non-specific binding of labeled specific binding partner in the capture and detection zones of the type of immunochromatographic strip under consideration.